1. Field of the Invention
The present invention relates to a switching power supply circuit for converting an AC power supply voltage to a DC power supply voltage, and more particularly to a switching power supply circuit preferably used as a standby power supply circuit and capable of protecting a switching element from a surge voltage.
2. Description of the Related Art
Most electronic devices, such as television sets, are provided with a main power supply circuit for operating each element in the main unit, and a standby power supply circuit for continuously operating standby circuits, such as a light receiving circuit for receiving a signal from a remote controller, a microcomputer, etc.
Standby power supply circuits usually have a rectifier circuit connected to an AC power supply, and continuously provide a relatively low power supply voltage (e.g. about 5V) for the above-mentioned light receiving circuit and microcomputer. When the remote controller has instructed the light receiving circuit to turn on the electronic device, the main power supply circuit is operated under the control of the microcomputer. On the other hand, when the remote controller has instructed the light receiving circuit to turn off the device, the operation of the main power supply circuit is stopped under the control of the microcomputer.
There is a recent tendency to use a switching power supply circuit as the standby power supply circuit. Switching power supply circuits usually comprise a rectifier circuit connected to an AC power supply, a smoothing capacitor for smoothing the output of the rectifier circuit, a converter transformer supplied with a DC current from the smoothing capacitor, and a switching element to which a DC current is supplied from the smoothing capacitor via the primary winding of the transformer. The ON/OFF of the switching element is controlled by the output of a switching control circuit. During the OFF period of the switching element, the energy accumulated in the above-mentioned primary winding is transmitted to the secondary winding of the transformer. The voltage generated at the secondary winding is rectified into a DC output voltage.
The ON/OFF cycle (duty ratio) of the switching element is controlled in accordance with variations in the DC output voltage, thereby stabilizing the DC output voltage of the power supply circuit.
If the switching power supply circuit is used as a standby power supply circuit, the capacitance of the smoothing capacitor is usually set small, compared to that of the main power supply. This is to reduce the size of the standby power supply circuit, and also to enhance the responsiveness with which the microcomputer detects whether the supply of power is restored, if it has failed for a short time.
If the smoothing capacitor has a small capacitance, a surge voltage may occur due to the reactance component of the AC power supply line when the AC power supply has been turned on, thereby significantly increasing the voltage across the smoothing capacitor. If the capacitance is large, the capacitor can absorb voltage surges. If, on the other hand, the capacitance is small, the capacitor cannot absorb a surge voltage, with the result that circuits connected to this capacitor may be affected by transient high voltages.
The switching control circuit incorporated in a switching power supply circuit includes an oscillation circuit, which oscillates when the terminal voltage of the smoothing capacitor exceeds a predetermined value after turn-on of the power supply. If the surge voltage is higher than the breakdown voltage of the switching element, the surge voltage may damage the switching element.
To prevent this, it is necessary to increase the capacitance of the smoothing capacitor so that it can absorb surge voltages. However, if this is done, the responsiveness of the microcomputer is reduced to low, therefore the microcomputer cannot quickly detect the restoration of power supply if the power supply has failed for a short time. If the main power supply circuit supplies an operation voltage to each element of the main unit before the microcomputer operates, the main unit may malfunction. Further, if a surge absorption element is inserted between the AC power supply and the rectifier circuit, current continuously flows through the surge absorption element, resulting in an increase in power consumption.
Japanese Patent Application KOKAI Publication No. 2002-51551 discloses a technique for varying the smoothing capacitance of a switching power supply circuit in accordance with the load, such that the smoothing capacitance is reduced in a standby mode, and the ON/OFF of a switching element is controlled only when the input voltage is lower than a predetermined value. This publication, however, contains no description concerning protection from a rapid increase in the voltage applied to the smoothing capacitor.
As described above, if a switching power supply circuit is used as a standby power supply, it is necessary to reduce the capacitance of the smoothing capacitor, but doing so, however, involves the risk that surge voltages may increase the breakdown voltage of the switching element and damage the element.